performance of lab-scale spac anaerobic bioreactor with high loading rate

CHINESE JOURNAL OF BIOTECHNOLOGYVolume 24, Issue 8, August 2008 Online English edition of the Chinese language journal

Cite this article as: Chin J Biotech, 2008, 24(8), 1413 1419.

Received: November 23, 2007; Accepted: February 25, 2008 Supported by: the Hi-Tech Research and Development Program of China (No. 2006AA06Z332), the Program for New Future Talents of Zhejiang Province (No. 2007G60G2010051). Corresponding author: Ping Zheng. Tel: +86-571-86971709; Email: [email protected]

Copyright © 2008, Institute of Microbiology, Chinese Academy of Sciences and Chinese Society for Microbiology. Published by Elsevier BV. All rights reserved.

RESEARCH PAPER

Performance of Lab-scale SPAC Anaerobic Bioreactor with High Loading Rate

Jianwei Chen, Chongjian Tang, Ping Zheng, and Lei Zhang

Department of Environmental Engineering, Zhejiang University, Hangzhou 310029, China

Abstract: The performance of a novel anaerobic bioreactor, spiral automatic circulation (SPAC) reactor, was investigated in lab-scale. The results showed that the average COD removal efficiency was 93.6% (91.1%~95.7%), with influent concentration increased from 8000 mg/L to 20 000 mg/L at 30°C and hydraulic retention time (HRT) of 12 h. The removal efficiency remained at 96.0%~78.7% when the HRT was shortened from 5.95 h to 1.57 h, as the influent concentration was kept constant at 20000 mg/L. The highest organic loading rate (OLR), the volumetric COD removal rate, and the volumetric biogas production of the SPAC reactor were 306 gCOD/(L·d), 240 g/(L·d), and 131 L/(L·d), respectively. In the cases of increasing influent COD concentration (from 8000 mg/L to 20000 mg/L), the effluent COD concentration was maintained at low level (852 mg/L for average) with volumetric COD removal rate increased by 162% and volumetric biogas production increased by 119%. In the cases of shortening HRT (from 5.95 h to 1.57 h), the volumetric COD removal rate and the volumetric biogas production were increased by 191% and 195%, respectively. The SPAC reactor shows good performances in adapting the continuous change of influent COD concentration and HRT.

Keywords: SPAC reactor, high organic loading rate, adaptability

Introduction

The anaerobic digestion process, which offers significant advantages such as biogas production, less sludge yield, lower energy consumption, and higher organic loading rate, has become increasingly attractive since the shortage of energy sources in the 1970s. This kind of technology has been widely applied in environmental and energy engineering[1,2].

In the past decades, the application of the third generation of anaerobic reactors such as IC (Internal Circulation) and EGSB (Expanded Granular Sludge Blanket) with characteristics of high mass transfer, high organic loading rate, and high system stability under loading shock, have promoted the development of anaerobic treatment[3]. It

is significant to further develop new type of anaerobic reactors with high efficiency since they are the key part of the anaerobic technology.

SPAC (Spiral Automatic Circulation) reactor is a novel reactor with high organic loading rate (OLR) invented by Zhejiang University[4]. The performance of SPAC reactor is reported in the article.

1 Materials and methods

1.1 Experimental reactor and inoculumThe schematic diagram of the SPAC reactor is shown in

Fig. 1. The reactor consisted of two polymethyl methacrylate columns with internal diameters of 0.14 m and 0.22 m, respectively. The lower part was the reaction zone

Jianwei Chen et al. / Chinese Journal of Biotechnology, 2008, 24(8): 1413–1419

with a working volume of 7.5 L, and the upper part was the settlement zone. Inside the SPAC reactor, there were the spiral channel and the automatic circulator, which differed from those of the IC and EGSB reactors. The hydraulic flow pattern of spiral channel was close to plug flow, and the effluent recycling in the automatic circulator helped to reduce the substrate concentration and balance the alkalinity.

The reactor was seeded with 7.5 L granular sludge (SS 51.2 g/L and VSS 41.5 g/L) taken from an IC plant treating pulping wastewater. The particle diameter ranged from 1.0 to 4.0 mm, with settling velocity ranging from 108 to 160 m/h.

1.2 FeedstockThe reactor was fed with synthetic wastewater consisting

of methanol, sodium acetate (in a ratio of 1:1 on COD basis), mineral materials, and trace solution (Tab. 1). Methanol and sodium acetate were added on demand. The pH was adjusted to 6.5–7.5 with HCl or NaOH. 1.3 Analytical methods

The chemical oxygen demand (COD) was measured by potassium dichromate in concentrated sulphuric acid for 2 h at 150°C. The suspended solids (SS) was measured by filtering the sample through a pre-dried filter paper. The

Fig. 1 Flow diagram of the SPAC reactor system 1: storage tank; 2: peristaltic pump; 3: SPAC reactor; 4: effluent; 5: gas

meter; 6: biogas

Tab. 1 Composition of the nutrient solution

Compound Concentration (mg/L) Compound Concentration

(mg/L)

Yeast extract 600 MgSO4 110

Beef extract 600 KH2PO4 5000

Tryptone 1800 K2HPO4 2000

NH4Cl 4000 Trace nutrient solution I (1)

CaCl2 220 Trace nutrient solution II (2)

(1) Trace nutrient solution I /g/L: EDTA 5.000, FeSO4 5.000; (2) Trace nutrient solution II /g/L: EDTA 15.000, H3BO4 0.014, ZnSO4·7H2O0.430, MnCl2·4H2O 0.990, CuSO4·5H2O 0.250, NaMoO4·2H2O 0.220, NiCl2 0.199, NaSeO4·10H2O 0.210

filter was dried at 105°C and then weighed after cooling. The volatile suspended solids (VSS) was measured by burning the previous filter at 550°C. The detailed process is described in reference [5].

The daily methane production was measured using a wet gas meter (BSD0.5). The biogas components were measured by a gas chromatograph (Angilent GC6890N). The pH value was measured by an electrode (pHS-9V).

2 Results and discussion

2.1 Start-up and operation of the reactor2.1.1 Start-up: After inoculation, the reactor was initially run with influent COD concentration of 3000 mg/L at HRT of 12 h. The OLR was progressively increased by raising the substrate concentration up to 8000 mg/L when the reactor reached steady states under which the COD removal efficiency was above 80% for at least three turnovers of the applied hydraulic retention time. Fig. 2 and Fig. 3 present the performance of the reactor during start-up. The COD removal efficiency reached beyond 90% after 9 days. On day 12, the OLR reached 16 g/(L·d), the volumetric COD removal rate reached 14.5 g/(L·d), and the volumetric gas production reached 8.1 L/(L·d), respectively. The gas yield of the removed COD was 603mL/g, and the CH4 content was 73%. The reactor was considered to be successfully started up since the reported OLR of the anaerobic reactor in lab-scale is between 10 and 20 g COD/(L·d)[6].2.1.2 Operation: In the following 11 days, the HRT was kept constant and the influent COD was raised up to 20 000 mg/L. Figs. 4–9 present the performance of the reactor during this period. Despite the increase of OLR up to 40 gCOD/(L·d), the effluent COD concentration was very low, 852 mg/L for average. The reactor showed a high average COD removal efficiency of 93.6%. The volumetric COD removal rate and the volumetric gas production were both elevated from 14.5 g/(L·d) to 38.5 g/(L·d) and 8.0 L/(L·d) to 18.6 L/(L·d), resulting in an average level of 26.7g/(L·d) and 13.6L/(L·d), respectively.

Fig. 2 Organic loading rate, volumetric COD removal rate and volumetric gas production during start-up


Fig. 3 Influent and effluent COD and removal efficiency during start-up

Fig. 4 Organic loading rate, volumetric COD removal rate and volumetric gas production during substrate concentration

increase

Fig. 5 Influent and effluent COD and removal efficiency during substrate concentration increase

The OLR was increased stepwise from 80 gCOD/(L·d) to 306 gCOD/( L·d), and the HRT was gradually reduced from 5.95 to 1.57 h. The HRT and OLR were directly related because the substrate concentration was maintained to be constant at 20 000mg/L. Figs. 10–15 show the influence of continuous HRT shortening on the reactor performance. The volumetric COD removal rate and the volumetric gas production increased from 77.5 g/(L·d) to

Fig. 6 Relationship between volumetric COD removal rate, volumetric gas production and influent COD during substrate

concentration increase

Fig. 7 Relationship between effluent COD, removal efficiency and influent COD during substrate concentration increase

240 g/(L·d) and 42.3 L/(L·d) to 131 L/(L·d), with 165 g/(L·d) and 86.3 L/(L·d) in average, respectively. The COD removal efficiency declined from 96.1% to 74.0% (effluent COD increased from 804 mg/L to 4266 mg/L) when the HRT was shortened from 5.95 h to 1.57 h. The volumetric COD removal rate continued to increase but gradually leveled out when the OLR was between 292 gCOD/(L·d) and 306 gCOD/(L·d), indicating that the system reached its highest capacity.

Recently, a few anaerobic reactors with high efficiency have been reported. Ren et al[7] obtained an OLR of 86 kgCOD/(m3·d) with the continuous-flow acidogenic reactors for hydrogen production. Li and Zuo[8] observed an OLR of 90 kgCOD/(m3·d) in anaerobic granular sludge bed reactor treating synthetic wastewater. Pu et al[9] reported the performance of EGSB treating sausage wastewater, whose OLR ranged from 220 kg COD/(m3·d) to 310 kg COD/(m3·d). When methanol wastewater was treated, Zhou and Ren[10] obtained an OLR of 26.8 kg COD/(m3·d), and Zhao[11] obtained an OLR of 37.19 kg COD/(m3·d). The OLR of SPAC reactor was as high as 306 gCOD/(L·d), which is among the top levels ever reported.


2.2 Adaptability of the reactor2.2.1 The adaptability to the continuous increase of the substrate concentration: As seen in Figs. 4–9, when the HRT was kept 12 h and the influent COD concentration was raised by 150% (from 8000 mg/L to 20 000 mg/L) within 11 days, the effluent COD fluctuated slightly. The COD removal efficiency increased by 5.05% (from 91.1% to 95.7%), the volumetric COD removal rate increased by 162% (from 14.6 g/(L·d) to 38.3 g/(L·d)), and the volumetric gas production increased by 119% (from 8.31L/(L·d) to 18.2L/(L·d)). The influence of the continuous increase of the substrate concentration on the reactor performance is slight.

Fig. 9 Relationship between effluent COD, removal efficiency and OLR during substrate concentration increase

2.2.2 The adaptability to the continuous HRT shortening: In the cases of keeping the influent COD constant at 20 000 mg/L and shortening the HRT by 73.6% (from 5.95 h to 1.57 h) within 19 days, the effluent COD increased and the COD removal efficiency dropped by 18% (from 96.0% to 78.7%, Figs. 10–15). Nevertheless, the volumetric COD removal rate and the volumetric gas production were raised by 191% and 195%, respectively. This may be owing to the quick response of the reactor to the HRT change.

Tab. 3 also presented the effect of continuous HRT shortening on the reactor performance in various reactors. The results showed that in terms of adapting the HRT reduction, the SPAC reactor is better than UASB, but worse than ABR. This may be owing to the considerably larger original HRTs of UASB and ABR. In fact, the acclimation of SPAC reactor is better than the other two. Fig. 10 Organic loading rate, volumetric COD removal rate

and volumetric gas production during HRT shortening

Fig. 8 Relationship between volumetric COD removal rate, volumetric gas production and OLR during substrate

concentration increase Fig. 11 Influent and effluent COD and removal efficiency

during HRT shortening

Tab. 2 Effect of continuous concentration increase on reactor performance

Reactor Initial concentration (mg/L)

Final concentration (mg/L)

Initial COD removal rate (%)

Final COD removal rate (%)

Sensitivityratio Sensitivity index References

SPAC 8000 20000 91.1 95.7 0.03 0.15 This study EGSB 6600 8300 82.4 71.4 0.51 5.7 [12] ABR 4002 6560 81.1 75.1 0.12 0.69 [13]


Tab. 3 Effect of continuous HRT shortening on reactor performance

Reactor Initial HRT(h) Final HRT(h) Initial COD removal rate(%)

Final COD removal rate(%)

Sensitivityrate Sensitivity index References

SPAC 5.95 1.57 96.00 78.70 0.24 4.24 This study UASB 12.00 6.00 93.00 69.00 0.52 12.40 [14] ABR 40.00 18.00 81.90 75.70 0.14 0.85 [13]

Fig. 12 Relationship between volumetric COD removal rate, volumetric gas production and HRT during HRT shortening

Fig. 13 Relationship between effluent COD, removal efficiency and HRT during HRT shortening

3 Conclusions

1) The SPAC reactor is a novel anaerobic reactor. The highest organic loading rate (OLR), the volumetric COD removal rate, and the volumetric biogas production of the SPAC reactor were 306 gCOD/(L·d), 240 g/(L·d), and 131 L/ (L·d), respectively. This is among the top level reported so far.

2) The SPAC reactor shows good performance in adapting the continuous change of influent COD concentration. In the cases of increasing influent COD concentration (from 8000 mg/L to 20 000 mg/L), the effluent COD concentration maintained at low level (852 mg/L for average) with volumetric COD removal rate increased by 162% and the volumetric biogas production increased by 119%.

3) The SPAC reactor also shows good performance in adapting the continuous change of HRT. In the case of shortening HRT (from 5.95 h to 1.57 h), the volumetric COD removal rate and volumetric biogas production were increased by 191% and 195%, respectively.

Fig. 14 Relationship between volumetric COD removal rate, volumetric gas production and OLR during HRT shortening

Fig. 15 Relationship between effluent COD, removal efficiency and OLR during HRT shortening

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performance of lab-scale spac anaerobic bioreactor with high loading rate

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